Arrangement for recording an IR-image

ABSTRACT

An arrangement for recording an IR image of an object. The arrangement has a focal plane array (FPA) of IR detectors and an optic which images the object on the focal plane IR detector array. At least one temperature reference is arranged in the beam path of the focal IR detector array. At least one IR reference detector is assigned as a reference among the IR detector elements in this array. Each temperature reference is coordinated with at least one of the IR reference detector elements so that each of the IR reference detector elements is impinged upon by radiation deriving essentially solely from one of the temperature references. An IR detector reference output signal is intended to be produced for each temperature reference. Each IR reference output signal is intended to set an individual reference level for the output signals from the remaining IR detector elements in the IR detector array.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an arrangement for recording anIR-image of an object.

DESCRIPTION OF THE RELATED ART

Hitherto, IR-cameras have been constructed with optical scanning systemsprovided with rotating and/or oscillating scanning components. Thesesystems scan an object and direct the beam path onto only a fewdetectors which are responsive to infrared light, these detectorsproducing a varying video signal which can be recorded on a TV-typescreen. In recent times, however, a new technique has been introduced inwhich image scanning is effected with a focal plane array of detectorsplaced in an image plane, instead of using movable components. Accordingto this new technique, each pixel itself creates an individual detectorelement. Although this technique was initially expensive, it has nowbegun to reach reasonable price levels. Neither is the techniquecompletely novel, since a number of FPA-systems are available, althoughit is primarily the different detector materials which distinguish fromone another. For instance, the detector elements may be pyroelectrical,Schottky Ptsi, InSb, MCT, PbSe, GaAs, etc. This type of technique isused particularly in military systems.

That which primarily distinguishes IR-cameras for civilian use,particularly those cameras retailed by Applicant's AGEMA INFRAREDSYSTEMS AB, from at least the majority of military systems, is that themilitary systems solely show a picture of the temperature distributionof scanned objects, whereas IR-cameras for civilian use also measure thetemperature at all pixels or picture elements.

Consequently, an IR-camera intended for civilian use will have higherdemands on absolute temperature accuracy than the military systems.Accordingly, the IR-camera has incorporated temperature references whichare moved automatically into the beam path of the detector or thedetector elements at least once with each image and the IR-camera isself-calibrating in accordance with these references.

The FPA-systems hitherto presented commercially include no actualtemperature references, and neither can they measure temperatureparticularly well. On the other hand, the FPA-systems often include adisc of uniform temperature which can be brought into the beam path,often automatically at given time points, so as to enable all of thedetector elements to be calibrated to deliver the same output signalwhen they look at object points that have the same temperature. Variantsof the FPA-system are also found in which it is possible to look withthe entire camera onto an external, heated disc, so as to enable theamplification from each individual detector element, i.e. each pixel, tobe calibrated.

However, the intention with the aforesaid temperature adjustingarrangements of FPA-systems is solely to ensure that the best possiblepicture or image is obtained, which is fulfilled when all detectorelements deliver an equally large output signal for an equally largeradiation signal. This intention has nothing whatsoever to do withmeasuring temperature.

SUMMARY AND OBJECTIONS OF THE INVENTION

The main object of the invention is to provide an which has temperaturemeasuring properties.

Another object of the invention is to render all of the detector elementoutput signals independent of detector temperature, long-term operation,the intrinsic temperature of the camera (background radiation), thetemperature of a disc inserted into the beam path at given time points,etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawings, in which

FIG. 1 illustrates schematically an optical array with a given beam pathof a first embodiment of the inventive arrangement;

FIG. 2 illustrates schematically an exemplifying embodiment of atemperature reference arrangement with associated optics in accordancewith the invention; and

FIG. 3 illustrates a third exemplifying embodiment of the inventivetemperature reference arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FPA-system illustrated in FIG. 1 includes an objective arrangementwhich is comprised of two lenses 1 and 2 which form an intermediatepicture or image 3 of an object to be examined. A relay opticalarrangement comprised of a system of lenses 4-7 generates an image on anarray 8 of IR-detector units 9, i.e. on an FPA-unit. It is to be notedthat both the objective optic and the relay optic are shown solely byway of example of suitable optics in an IR-camera, and that other typesof optics can be used, for instance mirror optics instead of lensoptics. Since this is well known to one skilled in this art and sincethe actual type of optic used has no significance with respect to theconcept of the invention, no other type of optic will be illustrated ordescribed.

In the embodiment illustrated in FIG. 1, there is shown two temperaturereferences 10 and 11 inserted in the intermediate image 3 with thepositioning indicated in the broken line square 3A, which shows theformat of the intermediate image. These temperature references areimaged on specific fields 12 and 13 respectively on the Ir-detectorarray 8, as illustrated in a plan view 8A. It will be seen from the planview that the fields 12 and 13 may include more than one detectorelement in the array, for instance four detector elements in the case ofthe illustrated embodiment. The object is imaged through the line system1-7, point-by-point on the remaining IR-detector elements in the arrayor group. Since an FPA-unit is cooled, the unit is seated in aDewar-vessel containing liquid nitrogen, behind a window 14 in a known,conventional manner.

In that case when each temperature reference 10, 11 is imaged on severalIR-detector elements, an average value is formed from the signalsobtained from the detector elements 12 which detect the temperature fromthe one temperature reference 10, and another average value is formed ofthe signals obtained from those detector elements 13 which detect thetemperature from the temperature reference 11. The temperaturereferences 10 and 11 will preferably have mutually differenttemperatures. The average value signals each set a respective referencelevel for the output signals from the remaining IR-detector elements inthe IR-detector array.

Adaptation to and scale division between the reference levels for thesignals from the remaining detector elements in the FPA-unit is effectedin a signal processor unit 15, to which the output signals of theIR-detector elements are applied. The signal processor unit 15 ispreferably a computer or microprocessor, e.g. of the 68000 series.FPA-units intended for use in systems are often sold together with aprocessor unit. The inventive arrangement involves a change of thesoftware of the associated processor unit.

The signal processor unit 15 delivers output signals to a display unit16, which illustrates a thermal image of the object while disclosingtemperatures or temperature ranges for different parts of the image.

It is also often convenient to be able to calibrate the detectorelements against one another. If a pyroelectrical detector array isused, it is necessary to modulate the pyroelectrical detector elements.In most cases, this is achieved with the aid of a so-called chopper,which has the form of a disc 17 of uniform temperature which is rotatedby means of a motor 19. A single point calibration is most often madeagainst this chopper, so as to obtain a mutually equal signal for alldetector elements. The disc is provided with at least one, often twodiagonally opposed apertures 18 adapted for the beam path of an imagefor detection by the FPA-unit. This is, in itself, a well-knowntechnique with FPA-units, but is also used in the present invention tocalibrate the detector elements against one another. Such a chopper isrequired for pyroelectrical detectors and should then be placed eitherbetween the lens 7 and the window 14, as shown in FIG. 1, or between thelenses 5 and 6. When using detectors of the type Schottky PtSi, InSb,MCT, GaAs, PbSe, etc., a chopper may also be placed close to the lens 1,since it is not fully necessary for the IR-reference detector elementsto be included by this calibration. It is only when the detectorelements are very similar to one another in the manufacturing processthat no calibration at all is required. In some instances, calibrationis necessary only in the initial stage of manufacture.

FIG. 2 illustrates an embodiment in which each of temperature references20, 21 are imaged by a respective concave mirror 22 and 23 on an imageplane 24 corresponding to the image plane 3 of the FIG. 1 embodiment,this image plane then being imaged on an FPA-unit (not shown) throughthe medium of a relay optic. Although the imagine plane 24 is shown asflat in the Figure, it will be understood that the plane may, inreality, be curved. The beam path from the temperature references iscollimated conveniently with some type of optical arrangement, in thepresent case illustrated solely by a diaphragm which gives a smallaperture angle. The concave mirrors are suitably provided with a tongue22' and 23', respectively, having a point which is tangential to theimage plane 24, so that a screening effect will be obtained between thebeam path between said mirrors 22 and 23 from the object and the beampaths from the temperature references 20 and 21. This minimizes the riskof detectors on the FPA-unit, which shall only be influenced byradiation from the temperature references, being influenced also byradiation from the object, and vice versa.

In the embodiment illustrated in FIG. 2, it is also shown that thetemperatures of the respective temperature references 20 and 21 can bemade variable in a controlled fashion. A drive circuit 25 is connectedto the temperature reference 20. The temperature of the reference 20 isdetected by a sensor 26 and the output signal of the sensor is deliveredto the drive circuit so as to adjust the temperature of the reference 20to an established value. A control signal which denotes a desired,established temperature value A is delivered to a control input on thedrive circuit 25, suitably from a computer, which carries into effectthe IR-camera control of incorporated circuits and which coacts withoperator manoeuvered control means (not shown) in a conventional manner.The temperature reference 21 is driven by another drive circuit 27 andhas a sensor 28 with the same functions as those described above. Thedrive circuit 27 is supplied with a control signal which denotes anotherdesired, established temperature value B. Control circuits of this kindare well known to the person skilled in this art and will therefore notbe described in detail here.

It is to be noted that one of the temperature references may be acooling element, which is thus cooled with the aid of the drive circuitand is servo-regulated to a desired temperature level. It is alsopossible to control the temperature references variably in time duringan imaging process, such that the temperatures of the references willincrease, e.g., ramp-wise, or have a sawtooth-like varying temperature.In such a case, only one temperature reference is actually required, forinstance only the reference 20 in FIG. 2. As will be understood, it isalso possible to use both a cooling element and a heating element withtime-varying temperatures as temperature references.

In the embodiment illustrated in FIG. 3, it is shown that a block 31 and32, each having one or more temperature references may be placedcompletely separate from a lens system 33 for the beam path 34 from theobject to the FPA-unit 35 positioned behind a window 30.

Furthermore, in the case of the illustrated embodiment, the IR-referencedetector elements 36, 37 of the FPA-unit may be placed separately fromthe IR-detector elements 38 for imaging the object. It applies ingeneral with all of the illustrated embodiments that the IR-referencedetector elements can either be included as an integral part of anIR-detector matrix or may be separate from the IR-detector matrix forimaging. The only essential criterion is that the IR-reference detectorelements are positioned so as to have the same attemperated environment.

The block 31 emits from at least one temperature reference (not shown) abeam path which is deflected by an oblique, flat mirror 39 onto theIR-reference detector elements 36 and focused at least in the vicinitythereof. The block 31 may include several temperature references, and anoptic arrangement in the block 31 may focus each of these referencesonto separate part-arrays of IR-reference detector elements 36A, 36B . .. 36N. Thus, there is obtained for each of these part-arrays a separatereference temperature level to which a signal processor unit (not shown)corresponding to the unit 15 in FIG. 1 can adapt the output signals fromthe remaining detector elements. The same applies to the block 32 andthe IR-reference detector elements 37.

The feature particularly illustrated in FIG. 3 is that the temperaturereferences need not necessarily be placed in an intermediate image, butmay instead be placed very close to the IR-reference detector elements.

It will be understood that many modifications are possible within thescope of the invention. The different properties and facilitiesillustrated in the different exemplifying embodiments can, in manycases, also be utilized in embodiments other than those illustrated anddescribed.

We claim:
 1. An arrangement for recording an IR-image of an object,comprising:a focal plane array (FPA) of IR-detectors; an optic whichimages the object on the focal plane IR-detector array; at least onetemperature reference (10, 11; 20, 21) arranged in a beam path to theIR-detector array; at least one IR-reference detector (12, 13; 36, 37)assigned as a reference among the IR-detectors in said array; each ofsaid at least one temperature reference being coordinated with at leastone of the at least one IR-reference detector, so that each of the atleast one IR-reference detector will be impinged upon by radiationderived substantially only from one of the at least one temperaturereference; an IR-reference detector output signal being produced foreach of said at least one temperature reference; each IR-referencedetector output signal setting an individual reference level for outputsignals from remaining IR-detectors in said IR-detector array.
 2. Anarrangement according to claim 1, wherein each said at least onetemperature reference has a mutually different temperature and isarranged in a predetermined position in the array.
 3. An arrangementaccording to claim 1, wherein said at least one IR-reference detector(12, 13) is integrated with the IR-detectors.
 4. An arrangementaccording to claim 1, wherein said at least one IR-reference detector(36, 37) is disposed separately from the remaining said IR-detectors insaid array.
 5. An arrangement according to claim 1, further comprisingat least one calibrating unit (17) having a specific temperature andintended for insertion into a beam path of all of said IR-detectors insaid array for calibrating the arrangement; and wherein all of saidIR-detectors in said array are calibrated against one another when saidat least one calibrating unit is inserted in the beam path, therebyproducing mutually equal output signals.
 6. An arrangement according toclaim 1, wherein a temperature of each of said at least one temperaturereference is variably selected.
 7. An arrangement according to claim 6,wherein the temperature of said at least one temperature reference isvariable in time in accordance with a predetermined pattern.
 8. Anarrangement according to claim 7, wherein said predetermined pattern isone of a ramp pattern and a sawtooth pattern.